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Remember the user has a "back" button on Android, however in this situation it sounds like they should either be saving or cancelling the form. "Back" might imply that you're leaving this menu but you can come back (as some apps DO let you do); cancel would imply you're clearing the data from the form, which I assume you're doing.

Making the back/cancel button look like part of the navigation might also suggest this effect; form entry cancel/save buttons are usually at the bottom. I would avoid the top bar unless you consistently have a top bar for navigation throughout the app; in which case you might even want to leave a back button on the top for all pages, and give an explicit cancel button for any forms.

As for save/okay I think they're both fine, depends on how you word your application I suppose; Save is more formal and makes more sense for a regular data entry perspective. "Ok" uses more common vernacular but I doubt either would confuse or imply anything incorrect.

Save/cancel seems to be the most semantically correct considering what I assume your application does. I wouldn't want users to question what happens when they click a button like "back" on a form, and I certainly wouldn't want them to mistakenly assume the data will be there on return.